DIVE-SHIELD

Under the microscope: Just a splash of seawater

Scoop up a bucket of seawater (or swallow a mouthful) and this is what you get: a bizarre menagerie of plants and animals, some of them known to us, others a complete mystery.

This extraordinary photograph shows a random splash of seawater, magnified 25 times. The Earth's open seas are home to countless tiny animals and plants that are known collectively as plankton.

1Crab larva Less than a quarter of an inch long, this delicate, transparent arthropod has a long way to go before it reaches maturity and yet its various body parts are already recognisable.
Tiny pincered claws can be clearly seen and so can a pair of vivid bug-eyes, their multi-faceted compound lenses just discernible.

2Cyanobacteria These coiled filaments are representatives of some of the most primitive life forms on Earth.Among the very first organisms to have evolved, cyanobacteria evolved a way of harnessing the power of sunlight to produce sugars - a process called photosynthesis, which liberates oxygen into the atmosphere.The billions of cyanobacteria in the oceans are responsible for producing a large proportion of oxygen to this day.

3Diatoms It is impossible to calculate how many individual diatoms are alive in the world's oceans at any one time - the number runs into quadrillions.
These small, boxy single-celled organisms are a type of algae, encased in a silica cell wall of great beauty. When they die, these tiny cell walls sink to the bottom of the sea, where they may be compacted to form rock.

4Copepods These bug-like creatures are the most common zooplankton (animal plankton) and may be the most important animals in the oceans, as they form the most abundant source of protein.
They are tiny, shrimp-like crustaceans, with a teardrop-shaped body and large antennae.
Copepods are energetic swimmers, have a well-developed nervous system and engage in vigorous attempts to evade capture.
They form the basic foodstuff of countless fish species. Some scientists believe that, taken together, copepods would form the largest single animal mass on Earth

5Chaetognaths These long, translucent organisms are arrow worms, predatory marine animals that form a large constituent part of the plankton.
For plankton they are big - an eighth of an inch to five inches in length.
They have a nervous system, two eyes, a mouth equipped with teeth and two tiny spines either side of their heads with which they grapple their prey (smaller plankton).
Some can even inject paralytic venom.

6Fish eggs Nearly all fish lay eggs, although a few (including some sharks) give birth to live young.
A few species protect and nurture their eggs (most notably seahorses, where the male takes on this role).
But most fish species release huge numbers of fertilised eggs into the open sea, a large percentage of which will be eaten.

7Marine worm A multi-segmented polychaete equipped with dozens of tiny, hair-like appendages with which it propels itself through the water.